The present invention is a method and apparatus that provides a system for improving the addressability and density of an ionographic printing image by crosstalk correction and ion beam positioning of individual pixel charges, using compensating modulation voltages, also known as "grey levels", applied directly to individual modulation electrodes in the print head.
Ionographic printing is that type in which charged particles are created at a remote location from the point at which a charge latent image is formed on a dielectric surface of a conductive substrate receptor. A stream of ions, assisted in some known systems by a fluid stream, generally an air stream, passes through a channel in the print head in which is placed a linear array of modulation electrode fingers, one for each pixel in a line of the printed image. Ions are created in a chamber, for example, by electrical discharge from a corona wire, maintained at very high positive voltage. During the printing process, the stream of ions is projected toward the receptor, usually a drum, which is placed at a high, negative electrical potential so as to attract the ions to its dielectric surface. Those ions which are projected from the printing head are pulled to the dielectric, over-coated surface of the receptor where they form a charge image, ready for immediate Xerographic-type development into a printed image. Application of low modulation voltages to the electrode fingers creates an electric field which can deflect the ions and locally eliminate them from the stream, producing varying densities of ions and thus varying density of the subsequently printed image. Three patents, U.S. Pat. No. 4,463,363, to Gundlach, et al., U.S. Pat. No. 4,524,371 to Sheridon, et al., and U.S. Pat. No. 4,644,373 to Sheridan et al., disclose different forms of a fluid jet assisted ion projection printing apparatus.
In ionographic printing, improvement in addressability and correction of the reduction in image density resulting from deflection of charges in the ion beam caused by redirection of the electric field associated with one of the modulation electrode fingers held at the modulation voltage, into an ion stream associated with an adjacent modulation electrode, referred to modulation electrode crosstalk interaction, is important in producing high-quality printed images. The crosstalk problem is very troublesome in binary and grey-scale printing. Modulation electrode crosstalk interaction is a phenomenon in which the electric field produced in adjacent, fully or partially modulated electrodes has the unwanted effect of locally eliminating ions from the projected stream, thus reducing the density of charges deposited on the receptor by the image-forming ion stream. Unless these crosstalk effects on the ion stream are corrected, unacceptable variations occur in the printed image of fine lines and edges.
Known systems have addressed the problem of controlling ion perturbation at the imaging surface, known as blooming, caused by the effect of previously deposited ions on the path of subsequent ions directed toward the imaging surface, by use of control electrodes positioned adjacent the ion stream path. U.S. patent application Ser. No. 07/636,326, entitled "In Situ Ionorgraphic Uniformity Correction" solves the problem of non-uniformity of image density by rapidly calibrating the ion current associated with each modulation electrode of a print array in an ionographic printing head using a simplified ion sensor which senses the ion current flow from groups of addressed modulation electrodes and adjusts the current in each to a uniform value during printing to produce high quality images of uniform density. U.S. patent application Ser. No. 07/633,883, entitled "Feedback Scheme For Ionographic Calibration" discloses a method and apparatus for producing uniform ion current which includes a calibration array, having electrodes corresponding to the modulation electrodes, positioned between the printing head and the receptor surface. The ion current sensed by each electrode of the calibration array is compared to a desired value of modulation voltage based on average modulation voltage and printing head characteristics, and a corrected modulation voltage is fed back to the corresponding modulation electrode in the printing head array, thereby adjusting the ion current across the corresponding modulation electrode to the uniform, calibrated value.
The problem of correcting ionographically printed images for crosstalk interaction has presented a major challenge to designers of ionographic devices. The development of a straightforward method of compensating for interaction between adjacent modulation electrodes in an ionographic printing head is a long-felt need and would represent a major technological advance in the ionographic printing field.